1. Field of the Invention
The present invention relates to an optical scanning apparatus, in particular, an optical scanning apparatus that allows a polygon mirror as a light deflector to reflect and deflect a beam emitted from light source means, and optically scans a surface to be scanned with the beam through a scanning optical system to record image information. The optical scanning apparatus is suitable, for example, for an image forming apparatus employing an electrophotographic process, such as a laser beam printer (LBP), a digital copying machine, or a multifunction printer.
2. Related Background Art
Up to now, in an image forming apparatus such as a laser beam printer or a digital copying machine, a light deflector composed of, for example, a rotary polygon mirror periodically deflects beams emitted from light source means including a semiconductor laser, for example, after being optically modulated according to an image signal. Subsequently, the deflected beams are converged into a spot shape on a photosensitive recording medium (photosensitive drum) surface using a scanning optical system (scanning lens system) having an fθ characteristic, and the recording medium surface is optically scanned therewith for image recording.
FIG. 19 is a main-part sectional view in a main scanning direction (main-scanning sectional view) of an optical scanning apparatus used in the conventional image forming apparatus of this type.
In FIG. 19, parallel beams emitted from a laser unit 91 including a semiconductor laser enter a cylindrical lens (condenser lens) 92 having a predetermined refractive power only in a sub scanning direction. The parallel beams incident on the cylindrical lens 92 outgo therefrom still in the form of parallel beams in a main-scanning section.
On the other hand, the above parallel beams are converged in a sub-scanning section into an image as a linear image elongated in the main scanning direction, around a deflection surface 93a of a light deflector 93 composed of a rotary polygon mirror. The beams reflected and deflected on the deflection surface 93a of the light deflector 93 are focused into an image in the form of light spot on a photosensitive drum 95 surface as a surface to be scanned by a scanning optical system (fθ lens system) 94 having the fθ characteristic. Thus, the photosensitive drum 95 surface is repeatedly scanned with the light spots. The scanning optical system 94 is constituted of a spherical lens 94a and a toric lens 94b. 
In the above optical scanning apparatus, a beam detector (BD) sensor 98 as a light detector is provided for adjusting a timing for the initiation of image formation on the photosensitive drum 95 surface prior to scanning of the light spot on the photosensitive drum 95 surface. The BD sensor 98 receives a BD beam constituting a part of the beams reflected and deflected by the light deflector 93, i.e., the beam at the time of scanning regions other than an image formation region on the photosensitive drum 95 surface, in other words, the beam that has not yet reached the image formation region. The BD beam is reflected by a BD mirror 96 and converged through a BD lens (condenser lens) 97 to enter a BD sensor 98. After detecting a BD signal (synchronous signal) from an output signal of the BD sensor 98, the timing for the initiation of image recording on the photosensitive drum 95 surface is adjusted based on the detected BD signal.
The photosensitive drum 95 is rotated in synchronization with a drive signal of the semiconductor laser inside the laser unit 91 at a constant speed while moving its surface in the sub scanning direction with respect to the scanning light spot.
An electrostatic latent image is thus formed on the photosensitive drum 95. The electrostatic latent image is developed using a known electrophotographic process and transferred onto a transferring material such as paper as a visualized image.
In general, a multiple-image forming apparatus employing the scanning optical system forms images in different colors in plural image forming parts, conveys the paper with conveying means such as a conveyor belt, for example, and multiply transfers the images onto the paper for the image formation. In particular, concerning the formation of a full-color image, which requires multi-color development, even a slight offset at the time of superposing the images leads to a deteriorated image. For example, regarding an image resolution of 400 dpi, even such a small offset as a fraction of 63.5 μm, which value corresponds to one pixel, causes a color drift or a color tint variation to extremely degrade the image.
To cope with the above, up to now, the same scanning optical system has been used for the color development, in other words, the optical scanning is performed with the same optical characteristics to reduce the risk of the image offset. However, this method involves a problem in that it takes a long time to output the multiple image or full-color image. With a view to solving the problems, there is a method of forming images by separate optical scanning apparatuses to obtain the images in respective colors and superposing the images on the paper conveyed by a conveying part.
However, this method involves a fear that the color drift is caused when superposing the images. As a method effective therefor, there is a method of detecting a position of the image and controlling the image forming part to correct the images according to the detection signal (see, for example, JP 01-281468 B).
Meanwhile, in the image forming apparatus for scanning plural photosensitive members with the beams, the same number of scanning optical systems as that of the photosensitive members are generally provided for forming latent images on the plural photosensitive members. In the apparatus, a problem arises in that the optical parts should be provided in a number corresponding to the number of scanning optical systems. In particular, the light deflector (polygon mirror) is an expensive optical part, resulting in increased costs. Also, the scanning optical system with a high-speed operation and a high definition particularly involves a large-sized light deflector as well as needs to have an ability to deflect the beam at a high speed, which causes a more serious problem.
To solve the above problem, proposed is an optical scanning apparatus for deflecting the plural beams with the common light deflector. Also in the optical scanning apparatus that scans the photosensitive member in the sub scanning direction using the common light deflector, it is necessary to provide a mechanism for shifting a scanning position of the beam in the sub scanning direction for improving a precision in superposing the images in the sub scanning direction. As a method of shifting the position, the deflection surface of the light deflector is selected, from which scanning with the beam starts in the sub scanning direction, so that the scanning position of the beam is shifted line by line in the sub scanning direction. The adjustment is thus performed.
Further, in recent years, there is an increasing demand for a compact, low-priced full-color image forming apparatus with a high image quality. To give an example of a system for meeting such a demand, a system employing a single, common light deflector for scanning with the plural beams, is proposed, in which a first lens is commonly used in the scanning optical system, and the beam is allowed to obliquely enter the deflection surface of the light deflector to reduce widths of optical members such as the lens and the polygon mirror in a height direction, thereby realizing cost-reduction as well as a thin, compact scanner.
However, the system allowing the beam to enter the light deflector in the sub-scanning section at an angle with its deflection surface, i.e., a so-called oblique incident optical system encounters problems in that the beam on an image plane (photosensitive drum surface) is turned and an imaging position of the beam varies in the sub scanning direction depending on the scanning position, i.e., a so-called scanning line curvature develops.
The scanning line curvature causes a problem in the LBP that requires high-precision printing. In particular, in the image forming apparatus for forming the full-color image through the multi-development by scanning the different photosensitive drums using a single polygon mirror, the scanning line curvature results in the color drift, which is undesirable.
Also, the turning of the beam undesirably hinders reducing a spot size; the size of the scanning spot should be minimized particularly in the optical scanning apparatus used in the high-definition image forming apparatus.